1. Field of the Invention
This invention relates to a method for nearly completely separating a tellurium component from tellurium-antimony-containing metal oxide catalysts.
2. Description of the Prior Art
Antimony-containing metal oxide catalysts containing tellurium, or to which tellurium is added, are used in various chemical reactions. For example, catalysts containing iron, antimony, vanadium, molybdenum, tungsten, tellurium, etc. (as disclosed in U.S. Pat. No. 3,668,147), catalysts containing uranium, antimony, vanadium, molybdenum, tellurium, etc. (as disclosed in Japanese Patent Publication No. 19764/72), catalysts containing tellurium and antimony (as disclosed in British Pat. No. 1,168,279), catalysts containing iron, antimony, tellurium, arsenic, cobalt, etc. (as disclosed in U.S. Pat. No. 3,900,426), are known to be useful for oxidation, ammoxidation, oxidative dehydrogenation, etc., of olefins.
These catalysts usually contain about 5 to 70 wt% of antimony. It is, therefore, industrially important to efficiently recover antimony from such waste catalysts. When the waste catalysts contain no tellurium, antimony can easily be recovered as metallic antimony or antimony trioxide by known techniques, such as a molten reduction method, a mix-roasting method using stibnite, etc. However, when antimony is recovered from tellurium-containing waste catalysts by such known techniques, the metallic antimony or antimony trioxide obtained is often contaminated by tellurium (in some cases, in a concentrated state), and it has thus been difficult to recover high purity metallic antimony or antimony trioxide.
For this reason, it is desirable to recover antimony after the separation of tellurium therefrom. Such techniques, however, require nearly complete separation and removal of tellurium from the waste catalysts. Therefore, it has been desired to develop a technique which is relatively simplified and permits complete removal of tellurium from such waste catalysts.
To recover antimony and tellurium by operations of dissolution and precipitation using acids, alkalis, etc., from the above-described catalysts as used in the oxidation, ammoxidation, oxidative dehydrogenation, etc., of olefins, the operations of dissolution and precipitation generally must be repeatedly applied because the catalysts usually contain a plurality of metallic components. This naturally makes such processes complicated and cumbersome. Furthermore, difficulties are encountered in the separation of tellurium from antimony and, therefore, in the recovery of antimony.
In view of these problems, an improved method has already been proposed, as described in Japanese Patent Application (OPI) No. 152819/77 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"). According to this method, efficiently recovering the tellurium component alone by bringing tellurium-containing catalysts in contact with reducing substances within a specific temperature range.
However, when the tellurium component is intended to be recovered from waste catalysts containing antimony, the method suffers from the disadvantage that it is difficult to sublimate and recover the tellurium component selectively relative to the antimony component. When the temperature at which the waste catalyst is brought in contact with the reducing substance is about 700.degree. C. or more, antimony trioxide sublimes together with the tellurium component. Therefore, difficulties arise in the separation of tellurium from antimony, and, consequently, in the recovery of tellurium. On the other hand, when the temperature is 700.degree. C. or less, if substances having high reducing capabilities are employed, the tellurium recovery ratio (% Te recovered, based on total Te content) is significantly lowered.
Although the reason for the latter phenomenon is not clear, it is believed that during the reduction, metallic antimony covers the surface of catalyst particles or is alloyed, making the sublimation of the tellurium component difficult. Thus, even by this proposed method, it is not always easy to isolate tellurium from waste catalysts.